The present invention relates to a dielectric filter device to be suitably used for telecommunications equipment such as a portable telephone set or a mobile telephone set.
To meet the demand for reducing the size and weight of telecommunications equipment including portable and mobile telephone sets, efforts have been made to produce miniaturized and flat dielectric filter devices to be used for such applications. Such devices are mostly so arranged that they can be directly mounted on printed circuit boards.
FIGS. 1 and 2 of the accompanying drawings illustrate a conventional tripolar interdigital type dielectric filter device which comprises a dielectric ceramic main body A having a substantially rectangular parallelepipedic profile and provided with three through bores B1, B2 and B3 running therethrough between a pair of oppositely disposed sides of the main body A. The through bores B1, B2 and B3 are provided with respective inner conductors C1, C2 and C3 on the peripheral surfaces thereof. The dielectric ceramic main body A has an outer surface portion carrying an outer conductor D. Further, the dielectric ceramic main body A has end surface portions (of which only one is shown in FIGS. 1 and 2 and indicated by E) as short circuiting surfaces each surrounding one of the the openings of each of the through bores C1, C2 and provided with a conductor layer for electrically connecting one end of each of the inner conductors C1, C2 and C3 with the outer conductor D. The other end surface portions (of which two are shown in FIGS. 1 and 2 and indicated by F) has open circuiting surfaces each surrounding the other of the openings of each of the through bores C1, C2 and C3 and provided with no conductor layer for electrically disconnecting the other end of each of the inner conductors C1, C2 and C3. The short circuiting and open circuiting surface portions are arranged in an interdigital manner. Input and output terminals G and H are arranged on the bottom of the dielectric ceramic main body A at locations adjacent the respective lateral inner conductors C1 and C3 and near the respective open surface portions F adjacent to the inner conductors C1 and C3. Finally, slits or grooves I1, I2, I3, and I4 are arranged on the top and the bottom of the dielectric ceramic main body A for interstage coupling in such a way that they run in parallel with each other and with the through bores, each being located between two adjacent through bores, so that they may regulate the degree of interstage coupling of the inner conductors that operate as a resonator.
The conventional dielectric filter device having a configuration as described above is then directly mounted on a printed circuit board by soldering the input and output terminals G and H to given input and output circuits provided on the printed circuit board.
With the conventional dielectric filter device as described above, desired input and output coupling capacities can be achieved for the device only by appropriately modifying the surface areas of the input and output terminals G and H. More specifically, as shown in FIG. 1 input and output terminals G and H having a large surface area are used if large input and output coupling capacities are required, whereas as shown in FIG. 2 input and output terminals G and H having a small surface area are used if small input and output capacities are required.
On the other hand, there are a number of electronic components, besides a dielectric filter device, that are to be mounted on a single printed circuit board. This means that only a limited area can be allocated to a dielectric filter device on a printed circuit board and the input and output circuits of the printed circuit board are dimensionally adjustable only within a narrow limit to meet the dimensional requirements of the input and output terminals of a dielectric filter device so that the input and output circuits of a printed circuit board have to be, more often than not, modified in a cumbersome way to meet the specific dimensional requirements of the input and output terminals of a dielectric filter device to be mounted thereon, providing an obstacle to efficient mass production of printed circuit boards. In other words, since dielectric filter devices have differently sized input/output terminals depending on the required input and output coupling capacities, printed circuit boards have to be provided with differently sized input and output circuits to accommodate the difference in the size of the input and output terminals of dielectric filter devices rise to cumbersome production control procedures.
In view of the above identified problem, it is therefore the object of the present invention to provide a dielectric filter device having input and output coupling capacities that are adjustable without modifying the profile and size of the input and output terminals.